The present invention relates to the pyrolysis of carbonaceous materials. More specifically, the present invention relates to the pyrolysis of carbonaceous materials involving a novel method of supplying heat or heat and pressure to the reaction.
Numerous processes in the chemical industry require substantial amounts of heat to bring about physical and/or chemical reactions. Normally such heat is supplied by process heaters, steam boilers and the like. Unfortunately, these conventional techniques result in substantial losses of heat. For example, stack losses alone amount to about 19% of the heat produced. In addition, there are normally losses in transmitting the heat from the point of production to the point of use, which in many cases can range from 3% to 20%. Additional losses occur where indirect methods of heating are employed, such as passing the material to be treated or the heat transfer medium through tubes or utilizing solid-to-solid heat transfer techniques. In addition, most such processes also require a maintenance of high pressures. Consequently, substantial additional energy is required in the compression of gases, etc. to maintain elevated pressures.
The above-mentioned energy requirements and energy losses, in supplying heat or heat and pressure, are particularly troublesome in processes for the pyrolysis of normally solid, carbonaceous materials. The anomaly in this situation is that the pyrolysis of carbonaceous materials is designed to recover materials which can be utilized as sources of energy, while at the same time present techniques for supplying heat or heat and pressure to the process consume substantial amounts of energy derived either from utilizing the products of the process itself or outside sources of energy.
Pyrolysis of oil shale is an outstanding example of the above. In the pyrolysis of oil shale, there are three basic heating methods. In "directly heated" processes, heat is supplied burning a fuel, which may be recycled retort off gas, with air (or oxygen) within the bed of shale. Depending on flow conditions, some portion of either the coke residue or the unretorted organic matter may be burned as well. In many designs, most, or even all the heat is provided by combustion of the kerogen or coke residue. In addition, such retorting produces significant amounts of hydrogen, the presence of which is beneficial to the retorting operation. For example, conducting the retorting operation in a hydrogen atmosphere serves to convert sulfur to hydrogen sulfide, thereby removing the same, and also to break down heavy materials by reforming. However, the in situ burning to produce heat for the process burns substantial amounts of the thus produced hydrogen and it is, therefore, necessary, if the desirable functions of hydrogen are to be retained, to supply make up hydrogen. The most usual process for producing hydrogen is by steam reforming of natural gas (first desulfurized, if high in sulfur) over a catalyst, such as a nickel catalyst. The reaction of the methane in water thus produces carbon monoxide and hydrogen. In order to obtain additional hydrogen, the effluent is subjected to a shift reaction over another catalyst, whereby the carbon monoxide and water react to produce carbon dioxide and additional hydrogen. Finally, the end product is treated in some manner to separate a concentrated stream of hydrogen. This process is a highly energy intensive process and contributes a substantial amount to the cost of operating a pyrolysis process. If oxygen is utilized as an oxidizing agent in the retorting operation, a separate operation to produce oxygen is necessary, which again is highly energy intensive. For example, a conventional operation is usually a cryogenic separation of oxygen from air involving liquifaction by the Joule-Thompson effect (throttling), expansion and vaporization. On the other hand, if air is utilized as an oxidizing agent, in the retorting operation, the off gas has a low caloric value (CV), for example, about 3.3 MJ/m.sup.3. The other two techniques for supplying heat in the retorting of shale are "indirectly heated" processes in which a separate furnace is used to raise the temperature of a heat transfer medium that is then injected into the retort to provide the heat. The two subclasses of indirectly heated retorts arise according to whether a solid or gaseous heat transfer medium is utilized. In the case of a gas, the shale is heated by gas-to-solid heat exchange. This is generally accomplished by recycling the off gas through a separate furnace to heat the gas by indirect heat exchange and then passing the hot gas to the retort. If a solid is utilized, heating is by solid-to-solid exchange. In this technique, at least a part of the spent shale or an inert material, such as ceramic balls, is heated in a separate furnace and the solid heat transfer medium is then transported to the retort. Both of the indirectly heated retorting techniques produce a medium CV off gas. It is obvious that all of these techniques require substantial amounts of energy and that substantial amounts of energy are also lost in utilizing such heating techniques.
The different types of oil shale also present their own peculiar processing problems which add to the problems of oil shale retorting. The two main types of oil shale include Green River or Mahogany zone (western) and Devonian or New Albany shale (eastern). Green River oil shale contains substantial amounts of carbonates in the form of dolomite and calcite, which are generally absent in the Devonian shale. The presence of carbonates presents a serious problem in the retorting of Green River Shale. Conventionally, liquid production is most rapid at about 425.degree. C. and the production of gases, namely, hydrogen and methane peaks at about 460.degree. C. Primary decomposition of liquids and gases is virtually complete at about 470.degree. C. Above 500.degree. C. secondary decomposition of char and gases takes place. However, at 500.degree. C. and higher, carbonate decomposition occurs which is detrimental to the retorting process. On the other hand, Devonian shale is low in hydrogen, compared to Green River shale, and high in sulfur, compared to Green River shale. Consequently, conventional low pressure retorting results in substantial reductions in the recovery of liquids and gases (about half that of Green River shale) and the liquid and gaseous products contain substantial amounts of sulfur. As previously indicated, retorting in a hydrogen atmosphere is beneficial in the processing of Green River shale to the extent that it reduces coking, aids in reforming heavy materials and removes sulfur. However, when utilized in the retorting of Devonian shale, it increases liquid and gas production where product recovery is essentially equal to that from Green River shale and removes sulfur in addition to reducing coking, etc. However, supplying sufficient hydrogen requires production of hydrogen by an energy intensive outside process, as well as additional costs for the compression of the hydrogen, since such hydropyrolysis is carried out at high pressures and it appears that the higher the pressure, the more beneficial the result. Consequently, additional energy is required for the compression of the hydrogen stream to maintain the high pressure in the retort.
While the above example discusses the problems and inefficiencies in the pyrolysis of oil shale, most of these problems are also present in the pyrolysis of other carbonaceous materials, such as coals, lignites, tar sands, biomass, etc.